DVE2: preliminary implementation of compressed states.

* iface/dve2/dve2.cc (dve2_compressed_state): New class.
(callback_context): Deal with general state*s, not dve2_state*s.
(transition_callback_compress): New function.
(dve2_kripke): Take a compress option.
(get_init_state, compute_state_condition, succ_iter,
format_state, state_condition): Handle compressed states.
(get_vars, compute_state_condition_aux): New helper methods.
* iface/dve2/dve2.hh (load_dve2): Add a compress option.
* iface/dve2/dve2check.cc: Add a -z option.
* iface/dve2/finite.test, iface/dve2/dve2check.test: Add more
tests.

iface/dve2/dve2.hh

@@ -61,6 +61,7 @@ namespace spot
 		    bdd_dict* dict,
 		    const ltl::atomic_prop_set* to_observe,
 		    const ltl::formula* dead = ltl::constant::true_instance(),
+		    bool compress = false,
 		    bool verbose = true);
 }